OA-6: Atlas V booster shortcomings due to MRCV anomaly

Despite successfully lofting the OA-6 Cygnus safely into orbit, United Launch Alliance (ULA) engineers have been investigating the first stage launch phase of the Atlas V’s March mission. The Atlas V booster shutdown several seconds ahead of schedule due to what is now classed as a fault with RD-180 Mixture Ratio Control Valve (MRCV) assembly.Atlas V Update:

The first mission to suffer an issue was the 2007 launch of the NRO L-30 spy satellite mission, which saw its pair of spacecraft deployed into a lower-than-intended orbit when the Centaur upper stage shut down ahead of schedule.

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The premature shutdown was blamed on a leaky valve, starving the upper stage of fuel for its second burn ahead of spacecraft separation. The spacecraft still went on to achieve their operational orbital parameters. As such, this mission is only classed as a “partial” failure by observers, while ULA class it as successful.

Interestingly, it was the Centaur that came to the rescue during the OA-6 mission to send the latest Cygnus spacecraft on a path toward the International Space Station.

The first stage was set to burn for four minutes and 15.5 seconds before its engine shut down. However, the timing of engine shutdown was shown to be six seconds earlier than planned.

Following staging, the Centaur’s RL10C-1 engine entered its prestart phase. Ten seconds after stage separation, the Centaur engine ignited to begin a single burn that was pre-planned for a duration of thirteen minutes and 38 seconds.

However, this burn lasted over a minute longer, as Centaur worked to overcome the shortfall of the Atlas V’s first stage performance.

Centaur ably delivered Cygnus into the correct 230km orbit and into its correct RAAN (Right Ascension of the Ascending Node), which ultimately classed the mission as a success.

The Upper Stage’s extra push resulted in Cygnus being 300 km further downrange than expected at spacecraft separation.

However, this parameter held no notable impact on the spacecraft’s mission, with any “in-track error” easily compensated for by adjusting the timing and duration of phasing burns to get to the rendezvous point.

“During the launch, the system experienced a premature first stage shutdown. Atlas is a robust system,” as outlined by ULA. “The Centaur upper stage compensated for the first stage anomaly, delivering Cygnus to a precise orbit, well within the required accuracy.”

While Centaur delivered Cygnus into her correct orbit, the stage still had one final job to complete: to deorbit itself into an area pre-noted in the NOTAM (Notice To Airmen) zone over the ocean near Australia.

The stage’s RL10C-1 started up as expected for the disposal burn, but due to the depletion of its propellent from having to burn longer than planned, it shut down shortly after the burn began.

As a result, the stage entered a contingency sequence for just such an occurrence but was unable to restart the engine due to “running on fumes”. It is understood, however, that Centaur still re-entered over water, south of New Zealand instead of south of (central to western) Australia.

A fiery farewell for a stage that showed it could overcome the challenges of an underperformance from its booster partner.

Since the issue, ULA engineers have been investigating the root cause of the booster’s underperformance and provided a specific update on Friday.

“The ULA engineering team has reviewed the data and has determined an anomaly with the RD-180 Mixture Ratio Control Valve (MRCV) assembly caused a reduction in fuel flow during the boost phase of the flight,” noted the company, pointing to the assumed issue with the fuel ratio causing the early shutdown.

“In addition to analysis and testing, all RD-180 engines are being inspected.”

The latter reference is in relation to the upcoming Atlas V missions, with the next launch set to loft the latest US Navy Mobile User Objective System (MUOS) spacecraft.

That mission remains without a confirmed launch target due to the investigation. The RD-180 hardware for the MUOS-5 launch is being inspected at the launch site, with the Atlas V inside SLC-41’s Vertical Integration Facility (VIF).

“Last Friday, in preparation for the MUOS-5 launch, the Atlas V completed the Launch Vehicle on Stand (LVOS) operation, erecting the Atlas V into the Vertical Integration Facility at Cape Canaveral Air Force Station,” added ULA.

“LVOS will allow configuration of the vehicle to support RD-180 engine inspections and confirm all engine components are ready for launch. The Atlas V MUOS-5 launch is targeted for early summer; a new launch date has not been secured on the Eastern Range. ”